Homologous recombination is a high fidelity protective mechanism used for maintenance of the genome. A well conserved family of helicases has been associated with different aspects of genomic stability. In humans there are five RecQ homologs, three of which are connected with diseases, BLM, WRN and RecQL4. The three syndromes associated with these genes, Bloom's, Werner's and Rothmund- Thompson's respectively, display diverse manifestations, but they all are associated with a hyper- recombination phenotype and high incidence of cancer. There appears to be a significant amount of functional overlap between these proteins in vitro and as such it remains difficult to determine the exact function of each individual in vivo. Saccharomyces cerevisiae has a single RecQ homolog, Sgs1, which can be partially complemented by at least two of its human orthologs, BLM and WRN. This makes it an attractive model system in which to study the role of RecQ helicases in homologous recombination. This project addresses the hypothesis that Sgs1 functions to dissolve recombination joints, which may avert and promote homologous recombination depending on the timing and specific circumstance. The project design will allow determination of the spatiotemporal association between Sgs1 and double-strand break repair in vivo. Furthermore, Sgs1 in its major functional form, the Sgs1-Top3-Rmi1 complex, will be purified for mechanistic in vitro studies using a variety of DMAsubstrates. In addition to DMA repair mechanisms, studies in cancer therapeutics are targeting telomere maintenance mechanisms. RecQ helicases have been shown to be involved in telomere biology as well as telomere maintenance mechanisms. G-quadruplexes, one of the structures proposed to exist at the telomere, are dissolved by Sgs1, WRN and BLM in vitro. As such molecules that stabilize G-quadruplexes and/or prevent resolution of these structures have been a focus of many anti-cancer therapies. Understanding the role of Sgs1 in homologous recombination may uncover more specific therapeutic target areas on which to focus in the future.